class TreeNode {
    public val: number;
    public left: TreeNode | null;
    public right: TreeNode | null;
    constructor(val?: number, left?: TreeNode, right?: TreeNode) {
        this.val = val === undefined ? 0 : val;
        this.left = left === undefined ? null : left;
        this.right = right === undefined ? null : right;
    }
}
